A solid-state foaming process is illustrated in FIG. 1 wherein foaming occurs while the polymer remains in the solid state throughout the foaming process. This process differs from other standard polymer foaming processes because the polymer is not required to be in a molten state. Generally, at the beginning of the method, the polymer is in equilibrium with the surrounding temperature and pressure so that the polymer is “unsaturated.” In block 102, the thermoplastic polymer is treated at an elevated pressure to cause the thermoplastic polymer to absorb gas. The treatment of the polymer in block 102 may be carried out in a pressure vessel, which is sealed, and then the material is exposed to a high pressure inert gas such as, but not limited to, carbon dioxide within the pressure vessel. The high pressure gas will then start to diffuse into the thermoplastic polymer over time, filling the polymer's free intermolecular volume. The gas will continue to saturate the polymer until equilibrium is reached. In block 104, the fully saturated polymer is removed from the saturation pressure to an environment of lower pressure so that the polymer is thermodynamically unstable, meaning that the polymer is supersaturated with gas and is no longer at equilibrium with the surrounding environment. The polymer will start to desorb gas from its surface into the surrounding environment. Desorption of some gas is desirable in some circumstances, for example, to avoid the creation of cellular structure in some areas of the polymer, such as at the surfaces. Desorption of the polymer can occur when the high-pressure gas is vented from the pressure vessel or the saturated thermoplastic polymer is removed into ambient atmospheric pressure. Heating of the partially saturated polymer in block 106 is carried out at a temperature below the melting temperature of the neat polymer. Heating produces a cellular thermoplastic polymer. Since the polymer is still in a solid state, the foams thus produced are called solid-state foams to distinguish them from foams that are produced in an extruder from a polymer melt. The cellular thermoplastic polymer is less dense than the noncellular polymer, thus saving material costs. However, depending on the polymer, the size of the cells, and relative density, the cellular polymer may or may not posses desirable characteristics.
In the last few years, “bio-based” solid poly(lactic acid) or “PLA” has been produced in large quantities for use in food and beverage packaging applications. This has led to a rise in the production of extruded solid PLA sheet or rod for thermoforming or molding food packaging and protective packaging that will biodegrade. Solid PLA articles use more materials and are heavier as compared to foams or cellular materials. Accordingly, it would be desirable to produce articles made from cellular PLA that can be subsequently thermoformed or molded similar to solid PLA.